Analog implementation of spread spectrum frequency modulation in a programmable phase locked loop (PLL) system

ABSTRACT

A PLL circuit is described. The PLL circuit includes: a signal generator; and a spread spectrum modulator coupled to the signal generator, where the spread spectrum modulator receives a control voltage as an input and provides a spread spectrum control voltage to the signal generator in response to the control voltage. In one embodiment, the spread spectrum modulator includes at least one selector, where the at least one selector selects a plurality of voltage levels that correspond to a spread mode and percentage of spread for the spread spectrum modulator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefits of U.S. Provisional ApplicationSer. Nos. 60/289,268 and 60/289,245, filed May 6, 2001, and entitled“Programmable Loop Bandwidth In Phase Locked Loop (PLL) Circuit” and“Phase Lock Loop (PLL) And Delay Lock Loop (DLL) Counter And DelayElement Programming In User Mode”, respectively.

This application is being filed concurrently with (1) the U.S. patentapplication of Gregory W. Starr and Wanli Chang for “Programmable LoopBandwidth In Phase Locked Loop (PLL) Circuit”, (2) the U.S. patentapplication of Gregory W. Starr, Yen-Hsiang Chang, and Edward P. Aungfor “Phase Locked Loop (PLL) And Delay Locked Loop (DLL) Counter AndDelay Element Programming In User Mode”, and (3) the U.S. patentapplication of Wanli Chang and Gregory W. Starr for “ProgrammableCurrent Reference Circuit”, and incorporates the material therein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to electronic circuits and, inparticular, to phase locked loop and delay locked loop circuits used inelectronic circuits.

2. Description of the Related Art

Consumer and commercial electronics must meet FCC electromagneticemissions standards. PLL circuits, like other electronic circuits, alsogenerate electromagnetic emissions that must meet FCC standards. In someexisting systems, this is accomplished by adding expensive and heavyshielding. In other existing systems, it is accomplished by implementinga digital spread spectrum technique where a clock signal has itsfrequency modulated in a controlled manner around a center frequency. Asnoted above, the shielding technique is expensive and physically heavy.On the other hand, the digital spread spectrum technique is rigid as itinvolves setting counters (or dividers) to one set of predeterminedvalues, and changing the counter settings to a second set ofpredetermined values to achieve a predetermined frequency modulation.

Another existing system, uses analog spread spectrum modulation ofcurrents. There are a number of disadvantages of using currentmodulation. One, it is difficult to generate a triangular waveform withcurrent modulation. Second, the output of the current modulation is acurrent which is not the most desirable parameter with which to controla voltage controlled oscillator.

The present invention addresses this and other disadvantages of existingcurrent reference circuits.

SUMMARY OF THE INVENTION

The present invention uses an analog approach to modulate the controlvoltage in a phase locked loop. One aspect of the analog approach of thepresent invention, unlike the existing digital approach, provides a morecontrolled modulation without having to resort to resetting counters tospecific predetermined values. Thus, the analog approach de-couples thecounters from the modulation, providing a more flexible modulationscheme. One aspect of the spread spectrum modulator of the presentinvention allows for easily changing the spread mode (i.e., the type ofspread) and the percentage of spread provided by the spread spectrummodulator. Another aspect of the spread spectrum modulator of thepresent invention provides for additional filtering that may be includedto reduce high frequency spurs. In another aspect, the spread spectrummodulator of the present invention provides spread spectrum modulationindependent of the process, supply voltage, and temperature.

The present invention encompasses a PLL circuit. In one embodiment, thePLL circuit of the present invention includes: a signal generator; and aspread spectrum modulator coupled to the signal generator, where thespread spectrum modulator receives a control voltage as an input andprovides a spread spectrum control voltage to the signal generator inresponse to the control voltage. In one embodiment, the spread spectrummodulator includes at least one selector, where the at least oneselector selects a plurality of voltage levels that correspond to aspread mode and percentage of spread for the spread spectrum modulator.

In one embodiment, the phase locked loop circuit includes a spreadspectrum modulator, where the spread spectrum modulator comprises avoltage divider and a selector coupled to the voltage divider, where theselector selects a plurality of voltages that correspond to a spreadrate and percentage of spread for the spread spectrum modulator. In oneembodiment, the selector includes a plurality of multiplexers, where afirst multiplexer of the plurality of multiplexers selects a highvoltage, a second multiplexer of the plurality of multiplexers selects alow voltage, and a third multiplexer of the plurality of multiplexersselects a reference voltage.

The present invention is explained in more detail below with referenceto the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of the PLL circuit of thepresent invention.

FIG. 2 is a circuit diagram of the analog spread spectrum modulator ofthe present invention.

FIG. 3 is graph of calculated voltages versus time for some of thevoltages designated in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises a PLL circuit with an analog spreadspectrum modulator. The following description is presented to enable anyperson skilled in the art to make and use the invention, and is providedin the context of a particular application and its requirements. Variousmodifications to the embodiments shown will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other embodiments and applications without departing from thespirit and scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles and features disclosedherein.

The present invention is primarily described and claimed with referenceto a PLL circuit. It is to be noted, however, that PLL and delay lockedloop (DLL) circuits are herein used interchangeably. Therefore,references herein to a PLL circuit, either in the description or claims,are not limited to PLL circuits but encompass DLL circuits as well.

FIG. 1 is a block diagram of one embodiment of the PLL circuit 100 ofthe present invention. In FIG. 1, the PLL circuit 100 includes a phasefrequency detector (PFD) 105, a charge pump (CP) 110 with a programmablecurrent reference circuit, an analog spread spectrum modulator 112, aloop filter 115 with a selectable bandwidth, a voltage controlledoscillator (VCO) 120, counter N 125, counter M 130, and counter O 135.

Also shown in FIG. 1, are shift registers 127, 132, 152, and 137, whichin one embodiment include D-type flip-flops. Shift registers 127, 132,and 137 are coupled to counter N 125, counter M 130, and counter O 135,respectively. In one embodiment, shift registers 152 is coupled to theCP 110, the loop filter 115, and the spread spectrum modulator 112. Inone embodiment, shift registers 152 are coupled to corresponding holdregisters of the CP 110, the loop filter 115, and the spread spectrummodulator 112.

The loop filter 115 with selectable bandwidth may also herein bereferred to as a loop filter with programmable bandwidth (orprogrammable bandwidth loop filter) or a loop filter with variablebandwidth (or variable bandwidth loop filter). The loop filter 115 withprogrammable bandwidth allows an effective shift in the open loopbandwidth of the PLL circuit. In one embodiment, the loop filter 115bandwidth is programmable in user mode using the shift registers 152.The U.S. patent application of Gregory W. Starr and Wanli Chang for“Programmable Loop Bandwidth In Phase Locked Loop (PLL) Circuit”, whichis filed concurrently with this application and is incorporated hereinby reference, describes such a loop filter with a programmablebandwidth.

Counters N, M, and O may also be referred to as dividers N, M, and O.The output of each of dividers N, M and O is equal to its respectiveinput divided by N, M, and O, respectively. In one embodiment, each ofN, M, and O are integers. In another embodiment, N, M, and O may benon-integers. In one embodiment, each of N, M, and O are equal to one.In another embodiment, the PLL may be without one or more of thedividers N, M, and O. In one embodiment, each of counters N, M, and Oand their associated delays may be programmed in user mode, i.e., theircount and delay settings may be programmed in user mode. The U.S. patentapplication of Gregory W. Starr, Yen-Hsiang Chang, and Edward P. Aungfor “Phase Locked Loop (PLL) And Delay Locked Loop (DLL) Counter AndDelay Element Programming In User Mode”, which is filed concurrentlywith this application and is incorporated herein by reference, describessuch counters.

In one embodiment, the CP 110 with a programmable current referencecircuit is programmable in user mode using the shift registers 152. TheCP 110 with a programmable current reference circuit is described ingreater detail in the U.S. patent application of Wanli Chang and GregoryW. Starr for “Programmable Current Reference Circuit” which is filedconcurrently with this application and is incorporated herein byreference. In another embodiment, a CP without a programmable currentreference circuit, but with a conventional current reference circuit,may be used in the PLL circuit 100 of the present invention.

The PFD 105 compares the feedback clock signal with a divided version ofthe reference clock signal, i.e., after the reference clock signal ispassed through divider N 125. Depending on the difference between thetwo signals compared by the PFD 105 (i.e., depending on whether the VCO120 needs to operate at a higher or lower frequency), either an up ordown signal is provided to the charge pump 110. In response, the chargepump 110 increases current supplied to the loop filter 115 or reducescurrent in the loop filter 15. As a result, a higher or lower controlvoltage (V_(CTRL)) is applied to the spread spectrum modulator 112. Thespread spectrum modulator 112 spread spectrum modulates the controlvoltage to produce the control voltage SS (V_(CTRL) _(—) _(SS)), aspread spectrum control voltage. The VCO 120 generates a signal (e.g., awaveform) whose frequency depends on the control voltage (or morespecifically, the control voltage SS).

FIG. 2 is a circuit diagram of the analog spread spectrum modulator 112of the present invention. Analog spread spectrum modulator 112 comprisesa buffer 210, a voltage level selector 220, a waveform generator 240, avoltage adder 260, and a voltage subtracter 270. The buffer 210 iscoupled to the voltage level selector 220 and the voltage adder 260. Thevoltage level selector 220 is in turn coupled to the waveform generator240 and the voltage adder 260. The waveform generator 240 and thevoltage adder 260 are both coupled to the voltage subtracter 270.

The buffer 210 comprises an amplifier 211, which in one embodiment is anoperational amplifier with unity gain. Buffer 210 is intended to preventexcessive loading on the control voltage. (It is to be noted, however,that in one embodiment, the buffer 210 may be excluded from the analogspread spectrum modulator 112.) The minus (or inverting) input terminalof amplifier 211 is coupled to the output terminal of amplifier 211. Thecontrol voltage is input to the plus (or noninverting) input terminal ofamplifier 211. The output of amplifier 211 is V_(A). In one embodimentwhere amplifier 211 has unity gain, V_(A) is simply a buffered versionof V_(CTRL). Thus, the following relation holds between V_(A) andV_(CTRL):V_(A)=V_(CTRL)  (Eqn. 1).

The voltage level selector 220 comprises a voltage divider 221 andmultiplexers 222, 223, and 224. Voltage divider 221 comprises a resistornetwork with a series of resistors, more specifically resistors 230,232, 234, 236, and 238. Node 231 is between resistors 230 and 232. Node233 is between resistors 232 and 234. Node 235 is between resistors 234and 236. Node 237 is between resistors 236 and 238. Voltage divider 221divides down the voltage V_(A). Accordingly, the voltages at nodes 231,233, 235, and 237 are progressively lower. Furthermore, each of thevoltages at nodes 231, 233, 235, and 237 is less than V_(A).

The voltages at nodes 231, 233, and 235 are used as inputs tomultiplexer 222. The voltages at nodes 233, 235, and 237 are used asinputs to multiplexer 223. The voltages at nodes 231, 233, 235, and 237are used as inputs to multiplexer 224. Each of multiplexers 222, 223,and 224 selects one of its inputs as an output. The selection is done inresponse to the select (SEL) signals applied to multiplexers 222, 223,and 224. In one embodiment, the selection may be done in user mode usingshift registers 152. The outputs of multiplexers 222, 223, and 224 areV_(H), V_(L), and V_(REF). Below are equations relating V_(H), V_(L),and V_(REF) with V_(CTRL):V _(H) =AV _(CTRL)  (Eqn. 2);V _(L) =BV _(CTRL)  (Eqn. 3);V _(REF) =CV _(CTRL)  (Eqn. 4);

-   -   where A is less than 1, B is less than A and less than 1, and C        is less than 1.

The values of A, B, and C, each depend on resistances of resistors 230,232, 234, 236, and 238. Additionally, their values depend on which ofthe input signals to multiplexers 222, 223, and 224 are selected to beoutput. The voltages V_(H), V_(L), and V_(REF) determine the spread modeand modulation range (i.e., percentage spread). Thus, the voltagedivider 221 in combination with the multiplexers 222, 223, and 224 andSEL signals set the spread mode, modulation range (i.e., percentagespread), and the reference voltage. It is to be noted that changing theSEL signals allows for changing the voltages V_(H), V_(L), and V_(REF),and, therefore, the spread mode and the percentage spread. In oneembodiment, the SEL signals are chosen by the user to achieve thedesired spread mode and percentage spread. In one embodiment, thisselection may be done in user mode using shift registers 152.

In the embodiment shown in FIG. 2, there are five resistors in thevoltage divider 221. In another embodiment, there may be a differentnumber of resistors in the voltage divider 221. For example, there maybe more than 5 resistors in the voltage divider. Having more resistorsin the voltage divider provides for a larger number of voltage levelsfrom which the multiplexers 222, 223, and 224 may select. This providesgreater flexibility in configuring (or programming) the analog spreadspectrum modulator 112 because it provides for a greater number ofoptions in selecting the voltages V_(H), V_(L), and V_(REF), and,therefore, the spread mode and percentage spread.

The waveform generator 240 receives V_(H) and V_(L) as inputs andprovides a voltage waveform V_(TRI) (which in one embodiment is atriangular voltage waveform) as an output at node 241. The waveformgenerator 240 comprises comparators 242 and 243, a set-reset flip-flop244, switches 245 and 246, current sources 247 and 248, and capacitorC_(LOAD) 249.

Comparator 242 compares V_(H) (received at the inverting input terminalof the comparator 242) with V_(TRI) (received at the noninverting inputterminal of the comparator 242) and provides an output to the reset (R)input terminal of the set-reset flip-flop 244. Accordingly, comparator242 provides a high output at the reset input terminal of the set-resetflip-flop 244 when V_(TRI) is greater than V_(H). Similarly, comparator242 provides a low output at the reset input terminal of the set-resetflip-flop 244 when V_(TRI) is less than or equal to V_(H).

Comparator 243 compares. V_(L) (received at the noninverting inputterminal of the comparator 243) with V_(TRI) (received at the invertinginput terminal of the comparator 243) and provides an output to the set(S) input terminal of the set-reset flip-flop 244. Accordingly,comparator 243 provides a high output at the set input terminal of theset-reset flip-flop 244 when V_(L) is greater than V_(TRI). Similarly,comparator 243 provides a low output at the set input terminal of theset-reset flip flop 244 when V_(TRI) is equal to or greater than V_(L).

In equations 2 and 3 above, which define V_(H) and V_(L), B is less thanA. Therefore, V_(H) is greater than V_(L). As a result, at any giventime V_(TRI) is not both greater than V_(H) and less than V_(L).Instead, V_(TRI) is usually between V_(H) and V_(L). In other words, itis less than or equal to V_(H) and greater than or equal to V_(L). Thus,most of the time, the outputs of both comparators 242 and 243 are low.Under this condition, the output of the set-reset flip-flop is notchanged. As such, one of switches 245 and 246 is closed, while the otheris open, and current is either being supplied to or sunk from node 241.

It is to be noted that supplying current to or sinking current from node241 involves supplying current (or charge) to or sinking current (orcharge) from capacitor C_(LOAD) 249. Thus, supplying current to orsinking current from node 241 is herein used interchangeably withsupplying current (or charge) to or sinking current (or charge) fromcapacitor C_(LOAD) 249.

If current is supplied to node 241, V_(TRI) is increased. When V_(TRI)is greater than V_(H), comparator 242 provides a high output at thereset input terminal of set-reset flip-flop 244. Moreover, when V_(TRI)is greater than V_(H), it is also greater than V_(L), and therefore, theoutput of comparator 243 to the set input terminal of set-resetflip-flop 244 is low. Accordingly, when V_(TRI) is greater than V_(H), Qis reset to low (or 0). As a result, switch 245 is opened and switch 246is closed. Thus, current from node 241 is drained by way of switch 246and current source 248. Draining current from node 241 decreasesV_(TRI). V_(TRI) is decreased until it becomes less than V_(L).

When V_(TRI) is less than V_(L), comparator 243 provides a high outputat the set input terminal of set-reset flip-flop 244. Moreover, whenV_(TRI) is less than V_(L), it is also less than V_(H), and therefore,the output of comparator 242 to the reset input terminal of set-resetflip-flop 244 is low. Accordingly, when V_(TRI) is less than V_(L), Q isset to high (or 1). As a result, switch 245 is closed and switch 246 isopened. Thus, current is supplied to node 241 from current source 247 byway of switch 245. Supplying current to node 241 increases V_(TRI).V_(TRI) is increased until it becomes greater than V_(H).

In the embodiment shown in FIG. 2, set-reset flip-flop 244 is used todetermine the states of switches 245 and 246 based on the outputs ofcomparators 242 and 243. In another embodiment, some other registerinstead of set-reset flip-flop 244 may be used to serve the function ofset-reset flip-flop 244. For example, in another embodiment, a D-typeflip-flop may be used in place of set-reset flip-flop 244.

In one embodiment, V_(H) is the high (or maximum) voltage of atriangular voltage waveform (i.e., V_(TRI) _(—) _(max)), V_(L) is thelow (or minimum) voltage of a triangle voltage waveform (i.e., V_(TRI)_(—) _(min)), and V_(REF) is the reference or base voltage of a trianglevoltage waveform. In other words, the following relations exist betweenV_(H), V_(L), V_(TRI) _(—) _(max), and V_(TRI) _(—) _(min):V _(TRI) _(—) _(max) =V _(H) =AV _(CTRL)  (Eqn. 5); andV _(TRI) _(—) _(min) =V _(L) BV _(CTRL)  (Eqn. 6).

It is to be noted that for a brief period of time V_(TRI) _(—) _(max)and V_(TRI) _(—) _(min) will be above V_(H) and below V_(L),respectively. As noted above, when V_(TRI) is greater than V_(H), Q isreset to low (or 0). As a result, switch 245 is opened and switch 246 isclosed. Thus, current from node 241 is drained by way of switch 246 andcurrent source 248. Draining current from node 241 decreases V_(TRI).V_(TRI) is decreased until it becomes less than V_(L). Similarly, asnoted above, when V_(TRI) is less than V_(L), Q is set to high (or 1).As a result, switch 245 is closed and switch 246 is opened. Thus,current is supplied to node 241 from current source 247 by way of switch245. Supplying current to node 241 increases V_(TRI). V_(TRI) isincreased until it becomes greater than V_(H).

In one embodiment, current sources 247 and 248 are programmable toprovide different current levels. In one embodiment, thisprogrammability is achieved by using a programmable current referencecircuit in conjunction with the current sources 247 and 248. In oneembodiment, current sources 247 and 248 are programmable in user modeusing shift registers 152. As noted above a programmable currentreference circuit is described in greater detail in the U.S. patentapplication of Wanli Chang and Gregory W. Starr for “ProgrammableCurrent Reference Circuit”, which is filed concurrently with thisapplication and is incorporated herein by reference.

Similarly, in one embodiment, capacitor C_(LOAD) 249 is programmable. Inone embodiment, the capacitor is programmable in user mode using shiftregisters 152. A programmable capacitor is described in the U.S. patentapplication of Gregory W. Starr and Wanli Chang for “Programmable LoopBandwidth In Phase Locked Loop (PLL) Circuit”, which is filedconcurrently with this application and is incorporated herein byreference.

The currents provided by and sunk by current sources 247 and 248,respectively, and the capacitance of capacitor C_(LOAD) 249 determinethe speeds at which V_(TRI) is increased to V_(H) or decreased to V_(L).The speeds at which V_(TRI) is increased to V_(H) or decreased to V_(L)determines the spread rate (i.e., the distance between two consecutiveV_(TRI) _(—) _(max)'S or V_(TRI) _(—) _(min)'s) of V_(TRI).

Voltage adder 260 (or summing amplifier 260) comprises amplifier 261(which in one embodiment is an operational amplifier) and resistors 262,263, 264, and 265. Resistor 264 is coupled to the output node 266 andthe inverting input terminal of the amplifier 261. Resistor 265 iscoupled between the inverting input terminal of the amplifier 261 andground. Both of resistors 262 and 263 are coupled to the noninvertinginput terminal of the amplifier 261. Resistor 262 is coupled to theoutput of multiplexer 224 which outputs V_(REF). In one embodiment,there may be a buffer, such as buffer 225 shown in FIG. 2, between theoutput of multiplexer 224 and resistor 262. Buffer 225 is intended toprevent loading multiplexer 224. Similarly, resistor 263 is coupled tothe output of amplifier 211 which outputs V_(A). As both V_(A) andV_(REF) are applied to the noninverting input terminal of amplifier 261via resistors 263 and 262, respectively, amplifier 261 combines V_(A)and V_(REF). The output voltage V_(B) of amplifier 261 is the sum ofV_(A) and V_(REF). In one embodiment, the resistances of resistors 262,263, 264, and 265 are selected such that there is a unity gain factorbetween V_(B) and the sum of V_(A) and V_(REF). In one embodiment, theresistance of resistor 262 is equal to that of resistor 263. Similarly,the resistance of resistor 264 is equal to that of resistor 265. It isto be noted that in another embodiment, some other relationship mayexist between these resistors. Using equations 1 and 4 above, thefollowing equation is derived for VB:V _(B) =V _(A) +V _(REF) =V _(CTRL) +CV _(CTRL) =V _(CTRL)(1+C)  (Eqn.7).

Voltage subtracter 270 (or differential amplifier 270) comprisesamplifier 271 (which in one embodiment is an operational amplifier) andresistors 272, 273, 274, and 275. Resistor 274 is coupled to the outputnode 276 and the inverting input terminal of the amplifier 271. Resistor275 is coupled between the noninverting input terminal of the amplifier271 and ground. Resistor 272 is coupled to node 241 (which provides thevoltage V_(TRI)) and the inverting input terminal of the amplifier 271.Resistor 273 is coupled to the output node 266 of the amplifier 261(which provides the voltage V_(B)) and the noninverting input terminalof the amplifier 271. As V_(B) and V_(TRI) are applied to thenoninverting and inverting input terminals of amplifier 271,respectively, via resistors 273 and 272, respectively, amplifier 271subtracts V_(TRI) from V_(B). The output voltage V_(CTRL) _(—) _(SS) ofamplifier 271 is the difference between V_(B) and V_(TRI). In oneembodiment, the resistances of resistors 272, 273, 274, and 275 areselected such that there is a unity gain factor between V_(CTRL) _(—)_(SS) and the difference between V_(B) and V_(TRI). In one embodiment,the resistance of resistor 273 is equal to that of resistor 272.Similarly, the resistance of resistor 275 is equal to that of resistor274. Moreover, the resistance of resistor 272 is equal to that ofresistor 274. It is to be noted that in another embodiment, some otherrelationship may exist between these resistors. Using equation 7 above,the following equation is derived for V_(CTRL) _(—) _(SS):V _(CTRL) _(—) _(SS) =V _(B) −V _(TRI) =V _(CTRL)(1+C)−V _(TRI)  (Eqn.8).

As noted above, in one embodiment, the resistances of resistors 262,263, 264, and 265 are selected such that there is a unity gain factorbetween V_(B) and the sum of V_(A) and V_(REF). Similarly, in oneembodiment, the resistances of resistors 272, 273, 274, and 275 areselected such that there is a unity gain factor between V_(CTRL) _(—)_(SS) and the difference between V_(B) and V_(TRI). In anotherembodiment, other resistance values may be selected so as to provide adesired non-unity gain factor.

In one embodiment, one or more of the resistors 230, 232, 234, 236, 238,262, 263, 264, 265, 272, 273, 274, and 275 is programmable. In oneembodiment, these resistors are programmable in user mode using shiftregisters 152. A programmable resistor is described in the U.S. patentapplication of Gregory W. Starr and Wanli Chang for “Programmable LoopBandwidth In Phase Locked Loop (PLL) Circuit”, which is filedconcurrently with this application and is incorporated herein byreference. The programmability of these resistors allows for selectingdifferent voltage levels for V_(H), V_(L), and V_(REF), without changingthe SEL signals of the voltage selector 220. It also allows greaterflexibility in selecting a spread mode and percentage of spread.Additionally, it allows for greater flexibility in selecting gainfactors for the voltage adder 260 and voltage subtracter 270.

In one embodiment, filter(s) may be added in the voltage path fromV_(CTRL) to V_(CTRL) _(—) _(SS) to reduce high frequency spurs inV_(CTRL). Such filter(s), for example, may be added by modifying thevoltage adder 260 and/or the voltage subtracter 270. For example, addinga capacitor, such as capacitor 267 shown in FIG. 2, to voltage adder 260would cause the voltage adder 260 to act as a low pass filter.Similarly, adding capacitors, such as capacitors 277 and 278 shown inFIG. 2, to voltage subtracter 270 would cause the voltage subtracter 270to act as a low pass filter.

Using equations 5, 6, and 8, the following equations are derived for themaximum value for V_(CTRL) (V_(CTRL) _(—) _(SS) _(—) _(max)) and theminimum value for V_(CTRL) (V_(CTRL) _(—) _(SS) _(—) _(min)):V _(CTRL) _(—) _(SS) _(—) _(max) =V _(B) −V _(TRI) _(—) _(min) =V_(CTRL)(1+C)−BV _(CTRL) =V _(CTRL)(1+C−B)  (Eqn. 9);andV _(CTRL) _(—) _(SS) _(—) _(min) =V _(B) −V _(TRI) _(—) _(max) =V_(CTRL)(1+C)−AV _(CTRL) =V _(CTRL)(1+C−A)  (Eqn. 10).

The spread spectrum modulator 112 allows the spread mode to be varied.There are three typical spread modes: down spread, center spread, and upspread. Examples of these three modes are summarized in Table 1 below.TABLE 1 Example of the various spread modes Non-Spread maximum minimumSpread Mode frequency % spread frequency frequency Down Spread 100 MHz0.5%   100 MHz  99.5 MHz Center Spread 100 MHz 0.5% 100.25 MHz 99.75 MHzUp Spread 100 MHz 0.5%  100.5 MHz   100 MHz

In Table 1, each of the spread modes has a non-spread frequency of 100MHz and a 0.5% spread. In the case of down spread, the maximum andminimum frequencies are 100 MHz and 99.5 MHz, respectively. In the caseof center spread, the maximum and minimum frequencies are 100.25 MHz and99.75 MHz, respectively. In the case of up spread, the maximum andminimum frequencies are 100.5 MHz and 100 MHz, respectively.

FIG. 3 is graph of calculated voltages versus time for some of thevoltages designated in FIG. 2. The graph in FIG. 3 include waveforms305, 310, 315, 320, 325, and 330 which represent voltages V_(B), V_(A),V_(CTRL) _(—) _(SS), V_(H), V_(TRI), and V_(L). In FIG. 3, thehorizontal axis represents time. The unit of time and the numbers on thetime scale depend on the spread rate of the triangular waveforms, i.e.,the distance or time between the occurrence of two consecutive peaks(highest values) in a triangular waveform.

The PLL circuit of the present invention may be used in many systems.For example, the PLL circuit may be used in a digital system. Morespecifically, the PLL circuit may be used in a digital system comprisinga programmable logic device (PLD), which as used herein also refers tocomplex PLD's (CPLD's). Additionally, the PLL circuit may be used in aPLD. In one embodiment, the PLL circuit is on the same die/chip as thePLD. As used herein a digital system is not intended to be limited to apurely digital system, but also encompasses hybrid systems that includeboth digital and analog subsystems. Thus, the present inventionencompasses digital systems that include the PLL circuit describedherein.

While the present invention has been particularly described with respectto the illustrated embodiments, it will be appreciated that variousalterations, modifications and adaptations may be made based on thepresent disclosure, and are intended to be within the scope of thepresent invention. While the invention has been described in connectionwith what are presently considered to be the most practical andpreferred embodiments, it is to be understood that the present inventionis not limited to the disclosed embodiment but, on the contrary, isintended to cover various modifications and equivalent arrangementsincluded within the scope of the appended claims.

1. A phase locked loop circuit comprising: a signal generator; and aspread spectrum modulator coupled to the signal generator, wherein thespread spectrum modulator receives a control voltage as an input andprovides a spread spectrum control voltage to the signal generator inresponse to the control voltage.
 2. The phase locked loop circuit ofclaim 1, wherein the spread spectrum modulator comprises at least oneselector, wherein the at least one selector selects a plurality ofvoltage levels that correspond to a spread mode and percentage of spreadfor the spread spectrum modulator.
 3. The phase locked loop circuit ofclaim 2, wherein the at least one selector selects a high voltage, a lowvoltage, and a reference voltage for the spread spectrum modulator. 4.The phase locked loop circuit of claim 3, wherein the at least oneselector selects the high voltage, the low voltage, and the referencevoltage for the spread spectrum modulator in user mode.
 5. The phaselocked loop circuit of claim 3, wherein the spread spectrum modulatorcomprises a voltage divider coupled to the at least one selector, the atleast one selector comprising a plurality of multiplexers, the voltagedivider comprising a plurality of resistors coupled in series, wherein afirst plurality of nodes from the voltage divider are coupled to a firstmultiplexer of said plurality of multiplexers, a second plurality ofnodes from voltage divider are coupled to a second multiplexer of saidplurality of multiplexers, and a third plurality of nodes from thevoltage divider are coupled to a third multiplexer of said plurality ofmultiplexers, further wherein the first, second, and third multiplexersoutput the high voltage, the low voltage, and the reference voltagerespectively.
 6. The phase locked loop circuit of claim 5, wherein thespread spectrum modulator comprises: a buffer coupled to the voltagedivider; a waveform generator coupled to the plurality of multiplexers;a voltage adder coupled to the plurality of multiplexers and the buffer;and a voltage subtracter coupled to the voltage adder and the waveformgenerator.
 7. The phase locked loop circuit of claim 6, wherein thewaveform generator comprises: a first comparator, wherein the firstcomparator compares a voltage at an output node of the waveformgenerator with the high voltage; a second comparator, wherein the secondcomparator compares the voltage at the output node with the low voltage;a flip-flop coupled to the first and second comparators; a first switchcoupled to a first output node of the flip-flop; and a second switchcoupled to a second output node of the flip-flop; a first current sourcecoupled to the first switch, the first current source for increasingcurrent at the output node; and a second current source coupled to thesecond switch, the second current source for sinking current from theoutput node.
 8. The phase locked loop circuit of claim 7, wherein thewaveform generator is programmable to provide different spread rates. 9.The phase locked loop circuit of claim 7, wherein the voltage addercomprises an operational amplifier, wherein the voltage adder receivesthe reference voltage and an output of the buffer at a positive inputnode and provides a voltage adder output that is a sum of the referencevoltage and the output of the buffer.
 10. The phase locked loop circuitof claim 9, wherein the voltage subtracter comprises an operationalamplifier, wherein the voltage subtracter subtracts the voltage at theoutput node of the waveform generator from the voltage adder output. 11.The phase locked loop of claim 1 further comprising: a detector; acharge pump filter coupled to the detector and the spread spectrummodulator; a loop filter coupled to the charge pump and the spreadspectrum modulator; a first divider coupled to the signal generator anda first input node of the detector, wherein the first divider receives asignal generator output signal from the signal generator and provides afirst input signal to the first input node of the detector; a seconddivider coupled to a second input node of the detector; a third dividercoupled to the signal generator; and wherein the second divider receivesa reference clock signal and provides a second input signal to thesecond input node of the detector, further wherein the third dividerreceives the signal generator output signal from the signal generatorand provides an output clock signal.
 12. The phase locked loop circuitof claim 1, wherein the spread spectrum modulator comprises a waveformgenerator.
 13. The phase locked loop circuit of claim 12, wherein thewaveform generator comprises: a first comparator, wherein the firstcomparator compares a voltage at an output node of the waveformgenerator with a high voltage; a second comparator, wherein the secondcomparator compares the voltage at the output node with a low voltage; aflip-flop coupled to the first and second comparators; a first switchcoupled to a first output node of the flip-flop; and a second switchcoupled to a second output node of the flip-flop; a first current sourcecoupled to the first switch, the first current source for increasingcurrent at the output node; and a second current source coupled to thesecond switch, the second current source for sinking current from theoutput node.
 14. The phase locked loop circuit of claim 12, wherein thespread spectrum modulator further comprises: a voltage subtractercoupled to the waveform generator; and a voltage adder coupled to thevoltage subtracter.
 15. A digital system including a programmable logicdevice and the phase locked loop circuit of claim
 1. 16. A programmablelogic device including the phase locked loop circuit of claim
 1. 17. Aphase locked loop circuit comprising a spread spectrum modulator, thespread spectrum modulator comprising a voltage divider and a selectorcoupled to the voltage divider, wherein the selector selects a pluralityof voltages that correspond to a spread rate and percentage of spreadfor the spread spectrum modulator.
 18. The phase locked loop circuit ofclaim 17, wherein the selector comprises a plurality of multiplexers,wherein a first multiplexer of the plurality of multiplexers selects ahigh voltage, a second multiplexer of the plurality of multiplexersselects a low voltage, and a third multiplexer of the plurality ofmultiplexers selects a reference voltage.
 19. The phase locked loopcircuit of claim 18, wherein the voltage divider comprises a pluralityof resistors coupled in series.
 20. The phase locked loop circuit ofclaim 19, wherein the spread spectrum modulator further comprises: abuffer coupled to the voltage divider; a waveform generator coupled tothe selector, wherein the waveform generator receives the high voltageand low voltage as an input and provides a waveform generator output ata waveform generator output node; a voltage adder coupled to the bufferand the selector, wherein the voltage adder receives the referencevoltage from the selector and a buffered version of a control voltagefrom the buffer, further wherein the voltage adder provides a voltageadder output voltage that is a sum of the reference voltage and thebuffered version of the control voltage; a voltage subtracter coupled tothe waveform generator, wherein the voltage subtracter receives thewaveform generator output and the voltage adder output, subtracts thewaveform generator output from the voltage adder output, and provides aspread spectrum control voltage as an output.
 21. The phase locked loopcircuit of claim 20, wherein the waveform generator comprises: a firstcomparator, wherein the first comparator compares the waveform generatoroutput with the high voltage; a second comparator, wherein the secondcomparator compares the waveform generator output with the low voltage;a flip-flip coupled to the first and second comparators; a first switchcoupled to a first output node of the flip-flop; and a second switchcoupled to a second output node of the flip-flop; wherein the firstswitch is coupled to a first current source for increasing current atthe waveform generator output node, and wherein the second switch iscoupled to a second current source for sinking current from the waveformgenerator output node.
 22. The phase locked loop circuit of claim 21,wherein the voltage adder comprises an operational amplifier, whereinthe voltage adder receives the reference voltage and an output of thebuffer at a noninverting input terminal, further wherein the voltagesubtracter comprises an operational amplifier, wherein the voltagesubtracter receives the waveform generator output at an inverting inputterminal and the voltage adder output at a noninverting input terminal.23. The phase locked loop of claim 20 further comprising: a detector; acharge pump filter coupled to the detector and the spread spectrummodulator; a loop filter coupled to the charge pump and the spreadspectrum modulator; a first divider coupled to the signal generator anda first input node of the detector, wherein the first divider receives asignal generator output signal from the signal generator and provides afirst input signal to the first input node of the detector; a seconddivider coupled to a second input node of the detector; a third dividercoupled to the signal generator; and wherein the second divider receivesa reference clock signal and provides a second input signal to thesecond input node of the detector, further wherein the third dividerreceives the signal generator output signal from the signal generatorand provides an output clock signal.
 24. A digital system including aprogrammable logic device and the phase locked loop circuit of claim 20.25. A programmable logic device including the phase locked loop circuitof claim
 20. 26. A method of providing an output clock signal, themethod comprising: spreading a control voltage utilizing an analogvoltage controlled spread spectrum modulator to provide a spreadspectrum control voltage; and generating an output clock signal inresponse to the spread spectrum control voltage.
 27. The method of claim26, wherein the spreading comprises: dividing the control voltage toprovide a plurality of voltage levels; selecting a high voltage, a lowvoltage, and a reference voltage from the plurality of voltage levels;generating a voltage waveform in response to the high voltage and thelow voltage; adding the reference voltage with a buffered version of thecontrol voltage to provide a sum voltage; and subtracting the voltagewaveform from the sum voltage to provide the spread spectrum controlvoltage.
 28. The method of claim 27 further comprising: comparing afeedback clock signal with a reference clock signal to provide thecontrol voltage.
 29. The method of claim 26, wherein the spreadingcomprises: generating a voltage waveform in response to a high voltageand a low voltage; adding a reference voltage with the control voltageto provide a sum voltage; and subtracting the voltage waveform from thesum voltage to provide the spread spectrum control voltage.
 30. A phaselocked loop comprising: means for spreading a control voltage, whereinsaid means for spreading uses analog voltage controlled spread spectrummodulation and provides a spread spectrum control voltage; and means forgenerating an output clock signal, wherein the means for generatinggenerates the output clock signal in response to the spread spectrumcontrol voltage.
 31. The phase locked loop of claim 30, wherein themeans for spreading comprises: means for dividing the control voltage toprovide a plurality of voltage levels; means for selecting coupled tothe means for dividing, wherein the means for selecting selects a highvoltage, a low voltage, and a reference voltage from the plurality ofvoltage levels; means for generating a voltage waveform coupled to themeans for selecting, wherein the means for generating a voltage waveformgenerates a voltage waveform in response to the high voltage and the lowvoltage; means for adding coupled to the means for selecting, whereinthe means for adding adds the reference voltage with a buffered versionof the control voltage to provide a sum voltage; and means forsubtracting coupled to the means for adding and the means for generatinga voltage waveform, wherein the means for subtracting subtracts thevoltage waveform from the sum voltage to provide the spread spectrumcontrol voltage.
 32. The phase locked loop of claim 31 furthercomprising means for comparing coupled to the means for generating andthe means for spreading, wherein the means for comparing compares afeedback clock signal with a reference clock signal, wherein the meansfor comparing provides the control voltage.
 33. The phase locked loop ofclaim 30, wherein the means for spreading comprises: means forgenerating a voltage waveform, wherein the means for generating avoltage waveform generates a voltage waveform in response to a highvoltage and a low voltage; means for adding, wherein the means foradding adds a reference voltage with the control voltage to provide asum voltage; and means for subtracting coupled to the means for addingand the means for generating a voltage waveform, wherein the means forsubtracting subtracts the voltage waveform from the sum voltage toprovide the spread spectrum control voltage.